1. Field of the Invention
The present invention relates to a developing apparatus which develops an electrostatic image formed on an image bearing member using an electrophotographic process, an electrostatic recording process, or the like, which is particularly used for a copier, a printer, a facsimile machine, or the like.
2. Related Background Art
In an image forming apparatus such as an electrophotographic copier, conventionally, there is known a developing apparatus according to a powder cloud method, a cascade method, a magnetic brush method, or the like as a developing apparatus to be applied to the image forming apparatus. These methods have different characteristics and are being put to practical use in various fields according to the respective characteristics.
Among these methods, in the case of the powder cloud method and the cascade method, a developing toner is concentrated on a part with a large electric field gradient of an electrostatic latent image formed on a surface of an image bearing member such as a photosensitive drum, that is, a part where an original image concentration is discontinuous, and a reproduced image to be obtained is emphasized in this part. Since a so-called edge effect is generated, these methods are advantageous in reproducing copy of a business document image such as characters, that is, line copy. However, the edge effect becomes a disadvantage in reproduction of a general gradation image (image including a halftone concentration), that is, reproduction of a part where a concentration of an original image changes continuously. Thus, these methods are not suitable for, for example, a full-color copier which is required of a high image quality.
In addition, in the case of both the powder cloud method and the cascade method, since an area where a developer acts on a photosensitive drum needs to be made larger, there is another disadvantage in that a volume of the developing apparatus itself increases.
On the other hand, in the case of the magnetic brush method of a two component development process, an image is formed by making a two component developer, which includes a magnetic carrier, a toner, and the like in a mixed form, to adhere to magnetic field generation means; causing the developer to stand like magnetic bead chains in a brush shape in magnetic pole parts; and then developing an electrostatic latent image on a photosensitive drum by rubbing the same. In this case, since the magnetic carrier itself in the developer functions as a soft development electrode, it is possible to deposit the toner in proportion to a charge density of the electrostatic latent image. That is, the magnetic brush method is suitable for reproduction of a gradation image. In addition, the magnetic brush method also has a characteristic that the developing apparatus itself can be constituted in a small size.
As further improvement of this magnetic brush developing apparatus of the two component development process, a magnetic brush development method using a developing sleeve serving as a developer carrying member is generally used. In order to attain an object of developing an electrostatic latent image on a photosensitive drum efficiently, a two component developer including a magnetic carrier, which is a powder of a magnetic material such as ferrite, and a toner with a pigment scattered in a resin is agitated to be mixed, and the toner is given charges by triboelectrification due to friction of the magnetic carrier and the toner. Meanwhile, this developer is held in a developing sleeve serving as a hollow cylindrical developer carrying member made of a nonmagnetic material having magnetic poles in the inside thereof. The developer is carried to a development area opposed to the photosensitive drum from a developer container by the developing sleeve, and the developer is caused to stand like magnetic bead chains by the action of the magnetic field in this development area to rub the surface of the photosensitive drum, whereby the electrostatic latent image formed on the photosensitive drum is developed.
This two component magnetic brush development method using a developing sleeve is used in many products such as a monochrome digital copier and a full-color copier which are required of a high image quality.
Up to now, in the case in which a rotation movement speed of a photosensitive drum is relatively low, that is, in the case of copier having a relatively low operation speed, a satisfactory developed image can be obtained even in a short development time and thus, it is sufficient to provide only one development sleeve. However, in the case in which the rotation movement speed of the photosensitive drum increases due to the recent tendency of requiring high operation speed of a copier, preferable image formation is not always performed with only one developing sleeve.
As a measure to cope with the situation, there is a method of increasing development efficiency by increasing a peripheral speed of the developing sleeve. However, if the peripheral speed of the developing sleeve is increased, a centrifugal force acting on the developer forming the magnetic brush increases causing a larger amount of developer to scatter, thus contaminating the inside of the copier, which would result in deterioration of functions of the apparatus.
As another measure, there is proposed a method of creating a repulsive magnetic field by arranging two magnetic poles of the same polarity side by side so as to be opposed to a development area and causing the developer to stand like magnetic bead chains under the created magnetic field in order to increase a width of the magnetic bead chains of a brush shape in the development area to develop an image more efficiently (Japanese Patent Application No. 49-116899; see Japanese Patent Laid-Open Application No. 51-43151).
In this method, as a magnetic pole arrangement in the developing sleeve for holding the developer for development and causing the developer to stand like magnetic bead chains in a magnetic pole part, and in particular an arrangement of development magnetic poles for causing the developer on the developing sleeve to stand like magnetic bead chains opposed to the photosensitive drum, it is extremely effective in terms of the development efficiency to provide the magnetic poles for forming the repulsive magnetic field.
However, in the repulsive magnetic field of the development poles opposed to the photosensitive drum, since a magnetic force for holding the magnetic carrier in the developing sleeve extremely decreases, the magnetic carrier deposits on the photosensitive drum to spoil an image quality, and preferable image formation cannot always be performed.
Thus, as a further measure to cope with the above-mentioned situation, a so-called multi-stage magnetic brush development method is being proposed in which two or more developer carrying members such as developing sleeves are arranged with peripheral surfaces thereof adjacent to each other to be placed side by side such that the developer is carried continuously through the respective peripheral surfaces, thereby extending a development time to increase development capability.
Here, FIG. 7 shows an example of the conventional developing apparatus of the multi-stage magnetic brush development process provided with two developing sleeves.
A developing apparatus 101 is provided with a developer container 2 arranged in parallel with a photosensitive drum 21. The inside of the developer container 2 is sectioned into a development chamber R1 and an agitation chamber R2 by a partition wall 3 parallel with the photosensitive drum 21. A toner storage chamber R3 is provided above the agitation chamber R2, and a supply toner 11 is stored therein. A toner 11 of an amount offsetting a toner consumed in development is dropped to be supplied into the agitation chamber R2 from a supply port 12 in the lower part of the toner storage chamber R3. On the other hand, a developer 13 in which the toner particles and magnetic carriers are mixed is stored in the development chamber R1 and the agitation chamber R2.
A carrying screw 4 is housed in the development chamber R1 and is rotationally driven to carry the developer 13 along a longitudinal direction parallel with the photosensitive drum 21 of the developer container 2. A screw 5 carries the developer in a direction opposite to the direction in which the carrying screw 4 carries the developer.
Openings are provided in the front side and the back side in the partition wall 3. The developer 13 carried by the screw 4 is received by the screw 5 from one of the openings, and the developer 13 carried by the screw 5 is received by the screw 4 from the other of the openings.
An opening portion is provided in a portion of the developer container 2 adjacent to the photosensitive drum 21. In the opening portion, two developer carrying members, namely, a first developing sleeve 6 and a second developing sleeve 9 formed of a nonmagnetic material are provided.
The first developing sleeve 6, which is opposed to the photosensitive drum 21 on an upstream side in a rotating direction a, of these two developer carrying members rotates in a direction of arrow b (direction opposite to the rotating direction a of the photosensitive drum 21) and is regulated to an appropriate developer layer thickness by a developer regulating member of a blade shape (layer thickness regulating blade) 8, which is arranged further upstream than a first development area 14 in the rotating direction of the developing sleeve 6, i.e., at an upper end of an opening of the developer container 2. Then, the first developing sleeve 6 carries the developer 13 to the first development area 14.
First magnetic field generation means (magnetic roller) 7 of a roller shape is fixedly arranged in the developing sleeve 6. This first magnetic roller 7 has a development magnetic pole S1 opposed to the first development area 14. A magnetic brush of a developer is formed by a development magnetic field which the development magnetic pole S1 forms in the first development area 14. This magnetic brush comes into contact with the photosensitive drum 21 rotating in the direction of arrow a in the first development area 14 to develop an electrostatic latent image in this first development area 14.
The first magnetic roller 7 has poles N1, N2, N3, and S2 other than the development magnetic pole S1. Among them, the poles N2 and N3 having the same polarity are adjacent to each other in the developer container 2, whereby a barrier is formed against the developer 13.
A second developing sleeve 9 serving as a second developer carrying member is rotatably disposed in a direction of arrow c in an area below the first developing sleeve 6, on the downstream side in the rotating direction a of the photosensitive drum 21, and substantially opposed to both the first developing sleeve 6 and the photosensitive drum 21. This second developing sleeve 9 is formed of a nonmagnetic material in the same manner as the first developing sleeve 6. A second magnetic roller 10 of a roller shape serving as second magnetic field generation means is arranged inside the second developing sleeve 9 in a non-rotating state. This second magnetic roller 10 has three poles, S3, S4, and N4.
As a flow of the developer 13, the developer 13 is carried through the poles N2, S2, N1, S1, and N3 in this order on the first developing sleeve 6 and then, the developer on the first developing sleeve 6 is moved to the second developing sleeve 9 to be carried through the poles S3, N4, and S4 in this order.
Among these magnetic poles, the pole N4 is in contact with the photosensitive drum 21 in a part where the second developing sleeve 9 and the photosensitive drum 21 are opposed to each other, that is, a second development area 15, where development is conducted for the second time on the electrostatic latent image on the photosensitive drum 21 which has passed the first development area 14. In this way, by performing the development for the second time, high development efficiency can be attained.
As described above, the developing apparatus is provided with two developing sleeves, whereby, for example, even if a development time is reduced following increase in a peripheral speed of a photosensitive drum, high development efficiency can be attained and image formation can be performed satisfactorily without causing a decrease in a development concentration and occurrence of concentration unevenness.
However, in the conventional example using the first developing sleeve 6 and the second developing sleeve 9, a magnetic attraction force works between the first magnetic roller 7 inside the first developing sleeve 6 and the second magnetic roller 10 inside the second developing sleeve 9, which is liable to cause a problem in that central parts of both the first magnetic roller 7 and 1 the second magnetic roller 10 bend.
As an example of a cause of the magnetic attraction force between the magnetic rollers 7 and 10, there is given such a fact that the opposed poles of both the magnetic rollers 7 and 10 (poles N3 and S3) are poles of opposite polarities. However, if these two poles are poles of the same polarity, since a magnetic field is not formed between both the poles and it is highly likely that delivery of a developer from the first developing sleeve 6 to the second developing sleeve 9 is not performed smoothly, these poles are often made opposite.
When the bending of the magnetic rollers 7 and 10 occurs, first, parts of the magnetic rollers 7 and 10 come into contact with inner walls of the developing sleeves 6 and 9 to generate a frictional force, which leads to requirement of a large force for development drive, causing, in some cases, the developing sleeves 6 and 9 to stop (lock) due to a too large load.
As a measure to cope with this problem, it is possible that a sufficient margin is secured between the outer diameters of the magnetic rollers 7 and 10 and the inner walls of the developing sleeves 6 and 9. However, in addition to using the two developing sleeves initially, if the developing sleeves are enlarged, this leads to an increase in size of the developing apparatus. This is inappropriate, for example, in the case in which four developing apparatuses are used as in a full-color copier, or in terms of marked demand for reduction in size of a copier and costs. Conversely, reducing an outer diameter of a magnetic roller means reduction in a volume of a magnet of the magnetic roller, which makes it highly likely that it becomes difficult to obtain a magnetic force suitable for development.
In addition, even if the sufficient margin can be secured and the outer diameter of the magnetic roller and the inner wall of the developing sleeve do not come into contact with each other, a problem described below is unavoidable. That is, if the magnetic roller bends, a distribution of a magnetic force on a surface of the developing sleeve becomes non-uniform, and coating of a developer on the developing sleeve becomes non-uniform. It is highly likely that such coating unevenness adversely affects an image.
Therefore, as a measure to cope with the above-mentioned problem, it is necessary to eliminate the bending itself. Usually, a magnetic roller is often manufactured with a magnet provided around a shaft core rod made of metal, and it is possible that a diameter of the shaft core rod is increased so as to enhance strength of the roller as means to cope with the bending. However, since increasing the diameter of the shaft core rod leads to reduction in a volume of a magnet part, it is highly likely that it becomes difficult to obtain a magnetic force suitable for development as in the above-mentioned case of reducing the outer diameter of the magnet.
In particular, if a developing sleeve with a small diameter of 25 mm or less is used, a diameter of the magnetic roller has to be reduced as well. Accordingly, the shaft core rod tends to be made as thin as possible in order to obtain a magnetic force. Thus, it is difficult to obtain a magnetic force suitable for development.